What is a Differential? How Differentials Work

A vehicle works by converting chemical energy in petrol into kinetic energy by means of combustion. The engine rotates a shaft which is connected to a gear-box; gears in this box determine how much of the power is sent to the wheels. Another shaft exits the gear box and is attached to a differentials box which then sends the power to each of the wheels.

Sending the power straight to the wheels from the gear box can cause difficulties. The wheels will not rotate at the same rate as each other when turning a corner, this is because they travel differing distances in the same space of time. They can be allowed to spin at different rates using a differential box.

Front wheel drive cars will have the differential box located at the front of the car, while rear wheel drive cars will have the box located at the back of the car. Four wheel drive cars will have three boxes, one for each set of wheels and one i in between the back and front sets of wheels.

The differential box is essentially a set of perpendicular toothed gears. The input shaft is attached to a gear at right angles two it sending changing the direction of the rotation. The simplest way in which it allows for the wheels to spin in different directions is by use of additional pinion gears attached to the gears at right angle. When the wheels spin faster these pinions allow for this whilst the other wheel continues at a slower speed.

This simple form of differential means that the same amount of torque (the rotational force) goes to each wheel, this is not always good. One wheel may have more grip than the other so may start to slip if the same torque is applied to it. This can damage internal components of the vehicle. A Limited slip differential is able to solve this problem.

A limited slip differential can solve this problem. By using a clutch mechanism it can direct differing amounts of torque to different sets of wheels. A spring is attached to the gears which forces them against a clutch plate if it starts to slip. A high amount of torque is needed to overcome this clutch plate so the torque form the engine is directed towards the wheel which is not slipping.

Essentially what differentials do is allow for wheels to travel at different speeds. This is needed for when a car is turning a corner or for when one wheel has greater grip than the other. Without this the wheel would be forced to travel at the same speed and would always be slipping, causing damage to car components. If you’ve been following my columns, you already have a good idea regarding how power is generated in your car and the course along which it travels. Eventually, the power reaches your car’s differential before it’s finally delivered to the wheels. Besides sending power to the wheels, your differential also reduces their speed and allows them to turn at different speeds. Today, we’ll take a closer look at this component. I’ll explain why your car, truck, or SUV needs one and how it works under the hood.

Why They’re Necessary

It’s a common misconception that a car’s wheels rotate at the same velocity. In truth, they often turn at different speeds. For example, consider making a right turn. Your car’s wheels on the inside of the turn will travel less distance than the wheels on the outside. Now, think for a moment. Your wheels are still intact at the end of the turn. That means the inside wheels rotated more slowly than those on the outside.

In vehicles with front-wheel drive, the differential isn’t needed to turn the rear wheels at different speeds. Likewise for rear-wheel drive vehicles and the front wheels, respectively. That’s due to the transmission turning both “driven” wheels and the other 2 basically following their lead. That said, the differential is necessary to rotate the “driven” wheels at varying speeds. Otherwise, they’d be locked and rotating in unison, precluding your car’s ability to make turns.

How They Work

They’re comprised of several parts, including a ring gear, side gear, pinion gears, axle shaft, input pinion, and pinion shaft. The pinion gears are contained within a “cage” surrounded by the ring gear. The input pinion rotates clockwise, causing the ring gear to rotate at an intersecting axis.

Remember, when your car turns, the inside wheels must travel more slowly than the outside wheels. Your differential accomplishes this by rotating the cage within the ring gear so the pinions can turn at different speeds. In effect, it is splitting your engine’s torque, thereby allowing the inside and outside wheels to rotate differently.

Torque And Traction

A full discussion of torque, traction, and how they interact is best left for a future column. But, it’s important to understand that a differential splits the engine’s torque, allocating it evenly to your car’s wheels. Traction plays a role in that the more of it there is in any given circumstance, the more effectively the torque can be delivered. Ultimately, that affects how well the differential does its job.

What I’ve described above is merely an overview of a powerful car part that performs a complex task. In a future column, we’ll explore this component in more detail.